How do we keep our dikes safe?
The water board manages approximately 1,500 km of dikes. Dikes gradually deteriorate over time, which reduces their safety. If a dike no longer meets safety standards, it must be reinforced.
How can a dike fail?
A dike can fail in several ways; we refer to these as failure mechanisms. The three main failure mechanisms in our management area are:
- Inward macrostability
This refers to the inner slope (the landward side of a dike). Instability causes the soil mass to lose its equilibrium, which can lead to sections of the dike sliding off. This is often caused by excessive groundwater pressure beneath the inner slope.
Height A dike must be high enough to keep water out. If water flows over the dike, it can cause flooding and damage to the dike.
's (precast) revetment Coastal and lake dikes often have stone revetments that protect the dike from waves and erosion. Damage to this revetment can cause erosion of the dike. This can ultimately lead to the dike failing.
How do we determine the strength of a dike?
The strength of a dike can be determined based on observations and calculations. Observations focus on signs of damage, such as cracks in the revetment, subsidence, and scouring. These signs of damage can affect the strength of the dike.
programme Safety 2030 programme
Through the programme 2030 programme , we aim to gain a continuous understanding of the current condition of our dikes, how they degrade over time, and the flood risks in various scenarios. We achieve this through a smart groundwater monitoring network and various analytical methods.
Groundwater Monitoring
The groundwater level within the dike plays a major role in the failure mechanism of inward macrostability. We have installed approximately 400 monitoring wells in the dikes. Together, they form our groundwater monitoring network. These monitoring wells continuously measure the groundwater level and automatically transmit the data.
Calculating safety
To determine how safe a dike is, we look at the forces acting on it (such as water pressure and waves) and the dike’s strength. If the forces acting on the dike exceed its strength, the dike may fail. If the dike is stronger than the forces acting on it, it will continue to function properly.
The strength of the dike depends, among other things, on the properties of the soil it is built from. That is why we conduct soil investigations. Samples of the dike soil are tested in the laboratory using various methods. Among other things, we measure soil strength and groundwater pressure.
We need this data to calculate how strong a dike is and whether it meets safety standards.
Remote sensing
In addition to measurements and calculations, we also use remote sensing—which literally means “observation from a distance.” Remote sensing is a general term for techniques we use to collect data on the geometry of the dike. We do this in various ways, such as using aerial photographs, satellite and drone imagery, or by hand.
Remote sensing allows us to detect damage to the surface of the dikes, such as cracks in the grass cover, loose stones, or areas of erosion. We can also identify unwanted vegetation on the dikes.
The images and measurements are compared with theoretical models of what a dike should look like according to safety standards. This allows us to detect deviations and carry out targeted inspections and maintenance.
Why are we doing this?
These various techniques and measurements provide us with real-time insight into the condition of our levees. This allows us to improve our assessments, carry out maintenance and management in a risk-based manner, and take timely action to prevent disruption and flooding.